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Union Syntax
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select_statement UNION ALL select_statement UNION ALL select_statement ...
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UNION is used to combine the result from multiple SELECT statements into a single result set. We Hive currently only support supports UNION ALL (bag union) i.e. , in which duplicates are not eliminated. The number and names of columns returned by each select_statement has have to be the same. Otherwise, a schema error is thrown.
If some additional processing has to be done on the result of the UNION, the entire statement expression can be embedded in a FROM clause like below:
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SELECT *
FROM (
select_statement
UNION ALL
select_statement
) unionResult
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For example, if we suppose there are two different tables that track which user has published a video and which user has published a comment, the following query joins the results of a union all UNION ALL with the user table to create a single annotated stream for all the video publishing and comment publishing events:
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SELECT u.id, actions.date
FROM (
SELECT av.uid AS uid
FROM action_video av
WHERE av.date = '2008-06-03'
UNION ALL
SELECT ac.uid AS uid
FROM action_comment ac
WHERE ac.date = '2008-06-03'
) actions JOIN users u ON (u.id = actions.uid)
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Unions can be used in views, inserts, and CTAS (create table as select) statements. A query can contain multiple UNION ALL clauses, as shown in the syntax above.
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In Hive 0.12.0 and earlier releases, unions can only be used within a subquery such as "SELECT * FROM (select_statement UNION ALL select_statement UNION ALL ...) unionResult". As of Hive 0.13.0, unions can also be used in a top-level query: "select_statement UNION ALL select_statement UNION ALL ...". (See HIVE-6189.) |